Polyvinyl Alcohol Lead Azide

The first cousin of British Service lead azide is the crystal form precipitated in the presence of polyvinyl alcohol PVA lead azide. At the present time, little (if any) PVA lead azide is produced in the United States, although it is believed 

As indicated in the introduction to this chapter, one of the major advances which affected the civil and military technology of lead azide occurred with the development of the dextrinated products. Today these continue to play the most significant role in civilian industry and are still used for some military applications. 

In Sweden the Forenade Fabriksverken of the Ministry of Industry utilizes more dilute solutions in making dextrinated azide. Although normally precipat-ing the azide at 60°C, this company has tried precipations from 75°C in (unsuccessful) attempts to reduce the HN3 content of the product and thus avoid the formation of copper azide in detonators fabricated from copper-containing metals. Azide contents up to 94% are obtained by their process. 

During the 1950s a spheroidal form of dextrinated lead azide was developed 

Dextrins. Dextrine, White Farina 30, the dextrin manufactured by the British firm of Elliot and Crabtree and used in the preparation of RD 1352, was supplied by Liang National Ltd. as Dextrine, White Farina required for manufacture of lead azide. There is no specification for this dextrin, and as received it contained 10.73% moisture. A portion was air-dried and was used to prepare the solutions for the three batches of dextrinated azide. 

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D.E.Seeger, PicArsn EDS Rept No 60 (1959) "Investigation of Polyvinyl Alcohol Lead Azide for Use in Detonators 24)R.L. Wagner, PATR 2662(1960), "Lead Azide,... 

Polyvinyl Alcohol Lead Azide (PVA Lead Azidej. Polyvinyl alcohol was used instead of dextrin in Germany [2]. In the U.S.A. it is in use in OUn Mathson Corporation [1311. The addition of 3% PVA makes lead azide of better initiating properties, higher density and an easier inflammability than dextrinated lead azide [1],... 

They also found that by adding small amounts of dextrin, polyvinyl alcohol or other hydrophilic polymers explosion could be prevented. It is known that these compounds are able to alter the crystal habit of several substances, including lead azide. 

RDX, etc) are too sensitive to be used per se and must be desensitized. For example LA may be desensitized by coating its crysts either with dextrin (See Dextrinated Lead Azide in Vol 1, pp A 46 to A548) or with PVAlc (polyvinyl alcohol) (See Vol 1, p A558-R). It can also be desensitized by controlling the size and shape of its crysts [See Service Lead Azide (Brit) in Vol 1, p A557-R], or by pptg it in colloidal form (See Vol 1, p A558-L)... 

Of the many types of lead azide—pure or basic, dextrinated, colloidal. Service, polyvinyl alcohol. Special Purpose, RD1333, RD1343, RD1352, and dextrinated colloidal—the types which have been used for military or commercial... 

Table Vll shows how the process used to manufacture lead azide, or the consequent product, significantly affects the quantity required to initiate a standard secondary explosive, RDX, in the stab-sensitive detonator (Figure la). Dextrinated lead azide has a lower output because it is less compressible and has more diluent namely, 8.5% dextrin compared to the 3.5% carboxymethyl cellulose (CMC) [34] in RD 1333, 2% polyvinyl alcohol in PVA lead azide, and no binder in Service lead azide (see Chapters 1 and 2). This situation is shown quantitatively in Table VII, which shows the minimum charge weights of each...

Lead azide exists as four polymorphic forms [287] of which the orthorhombic oc-lead azide is the most stable [276]. In fact, a-lead azide is the only acceptable form for technological applications. Presently, the state of the art of making the polymorphs can be summarized as follows a-lead azide is the main product of precipitation, with traces of the other forms present [288]. The monoclinic [276] /3-form is stable when dry, but re crystallizes as the a-form [276,289]. The presence of organic dyes (eosin, neutral red) at the precipitation enhances formation of /3-lead azide [276] the presence of hydrophile colloids inhibits it [276,287]. The monoclinic [287] 7-form, apparently less stable than a and /3 [289], is obtained from pure reagents at a pH of 3.5-7 [287] or in the presence of polyvinyl alcohol [289]. The triclinic 6-form precipitates from pure reagents between pH 3.5 and 5.5 [287]. No method is presently available to yield a single polymorph exclusively, but the crystals differ sufficiently in shape to allow hand selection under the microscope [287,288]. 

A second monoclinic modification (7-lead azide) and a triclinic modification (5-lead azide) have been identified (Chapter 3). These modifications form by changing the pH of the growing solution and by doping. It is found that polyvinyl alcohol, added to 5% solutions of ammonium acetate, favor the gamma modification when slow cooled below 40°C [21]. Other additives favor the formation of other polymorphs for example, dextrine favors the a-polymorph and eosin, the j3-polymorph [21]. These latter polymorphs are reported to be more light sensitive than either the alpha or beta polymorphs. 

There are also reports that mixing of DNA with polymers leads to formation of hydrogel by crossUnking. When photo-crosslinkable polyvinyl alcohol (azide-unit pendant water-soluble photopolymer AWP) was mixed with DNA and the resulting mixture irradiated with ultraviolet light, a hydrogel film was formed, due to photo-crosslinking of DNA and AWP, that shows expansion and contraction in response to the media in which it is soaked. It expands in pure water whereas it... 

War Department Field Manual FM 9-40, Ordnance Unexploded Bombs Organization and Operation for Disposal (1943) states for lead azide, Sensitivity. . . varies with crystal size. .. Large crystals detonate spontaneously. Sensitivity concerns over crystal size is compatible with the manufacturing process for lead azide in which dextrin or polyvinyl alcohol is added to inhibit crystal growth during the precipitation stage. (See Explosives by Rudolph Meyer.) FM 9-40 also states for TNT, Impurities may cause exudation. This is undesirable because of probabihty of low order detonation and increased sensitivity to shock of the explosive. ... 

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